Objective : This study was designed to investigate the effect of Hyeolbuchukeo-tang (HCT) on NO production and the molecular mechanism of NO production modulated by HCT in the primary VSMC (vascular smooth muscle cells). Method : Primary VSMC was established from aorta and cultured VSMC used in this study. NO production of VSMC was assayed by Griess reagent and the expression of iNOS gene was assayed by Western, RT-PCR. Result : $TNF-{gamma}$ induced NO production, but $IFN-{gamma}$ or HCT alone did not induce NO production in cultured VSMC. However, $IFN-{gamma}$ or HCT potentiated NO production in $TNF-{gamma}-treated$ VSMC in a time- and dose-dependent manner. $TNF-{gamma}$ induced the iNOS gene expression corresponding to NO production in $TNF-{gamma}-treated$ VSMC. HCT potentiated NO production in $TNF-{gamma}-treated$ VSMC by about 20%, but HCT did not increase the level of iNOS mRNA in $TNF-{gamma}-treated$ VSMC. HCT slightly increased the level of iNOS protein in $TNF-{gamma}-treated$ VSMC. Calcium ionophore A23187 decreased NO production in $TNF-{gamma}-treated$ VSMC, but HCT attenuated the effect of A23187. Conclusion : As NO is deeply involved in the development of arteriosclerosis and dilation of blood vessels, drugs or chemicals modulating NO production in VSMC could be used for preventing and treating arteriosclerosis. Considering the effect of HCT on the modulation of NO production in VSMC, MCT has a potential capacity for preventing and treating diseases of the circulation system including arteriosclerosis.